Mitsubishi 8M21 Specifications
| Engine Model: Mitsubishi 8M21 |
| Engine Type: 4-cycle 8-cylinder V-block OHV 16-valve liquid-cooled diesel |
| Total Displacement: 1294 cu.in (21.2 L) |
| Rated Engine Power: 370-430 Hp (272-316 kW) at 2000 rpm |
| Fuel System Type: Direct injection |
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The Mitsubishi 8M21 is a 21.2-liter, 90-degree V8
naturally aspirated diesel engine that served as a heavy-duty powerplant within
the Mitsubishi Fuso commercial vehicle lineup. Introduced in the mid-1990s, this
engine was engineered to deliver the substantial torque output and long-term
durability demanded by heavy-duty truck and luxury coach applications. It was
primarily utilized in the Mitsubishi Fuso Super Great series of heavy trucks and
the Aero Queen and Aero Ace luxury coaches, where its smooth power delivery and
robust construction were particularly valued.
The 8M21 belongs to the M-series family of Mitsubishi diesel engines, which
shared a square bore and stroke configuration - 150 mm by 150 mm - across
multiple displacements. This architecture, characterized by a cast iron cylinder
block, overhead valve (OHV) valvetrain with two valves per cylinder, and direct
injection combustion system, prioritized mechanical simplicity and
serviceability. The naturally aspirated configuration eliminated the complexity
of turbocharging while providing sufficient power for the vehicle applications
of its era. The 8M21 was produced in multiple power variants, with outputs
ranging from 370 to 430 horsepower depending on the specific calibration and
application.
The 8M21 is a four-stroke, water-cooled, naturally aspirated diesel engine
configured for longitudinal mounting in rear-wheel-drive truck and bus chassis.
Its 90-degree V8 configuration provides inherent primary and secondary balance,
contributing to smooth operation in passenger transport applications.
Engine Type: 90° V8, 4-cycle, water-cooled, naturally aspirated diesel.
Displacement: 21,205 cc (1,294.0 cu.in).
Bore and Stroke: 150.0 mm (5.91 in) x 150.0 mm (5.91 in).
Compression Ratio: 18.0:1.
Valvetrain: OHV, 16-valve (2 valves per cylinder).
Fuel System: Direct injection with in-line injection pump.
Aspiration: Naturally aspirated.
Firing Order: 1-2-7-3-4-5-6-8.
Rated Speed: 2,200 rpm.
Power Rating
Maximum Power (8M21-1): 272 kW (370 Hp) at 2,200 rpm.
Maximum Power (8M21-2): 294 kW (400 Hp) at 2,200 rpm.
Maximum Power (8M21-3): 309 kW (420 Hp) at 2,200 rpm.
Maximum Power (8M21-4): 316 kW (430 Hp) at 2,200 rpm.
Torques
Maximum Torque (8M21-1): 1,274 Nm (940 lb-ft) at 1,200 rpm.
Maximum Torque (8M21-2): 1,392 Nm (1,027 lb-ft) at 1,200 rpm.
Maximum Torque (8M21-3): 1,470 Nm (1,084 lb-ft) at 1,300 rpm.
Maximum Torque (8M21-4): 1,519 Nm (1,121 lb-ft) at 1,200 rpm.
The cylinder block is manufactured from high-strength
cast iron using a deep-skirt design, where the cylinder walls extend below the
crankshaft centerline to provide maximum structural rigidity under the high
cylinder pressures of diesel operation. The block features a 90-degree V-angle
with a closed-deck configuration, ensuring the cylinder bores are fully
supported by the water jacket structure at the top deck surface for superior
head gasket sealing.
Pressed-in, replaceable wet liners are employed, allowing for engine rebuilding
through liner replacement - a critical feature for commercial vehicles intended
for high-mileage service lives. The block incorporates five main bearing webs
per cylinder bank, with four-bolt main bearing caps on the center journals for
increased bottom-end strength. The deep-skirt design also provides robust
mounting points for the heavy-duty transmission and engine mounts required in
truck and bus applications.
Block Material: Cast iron.
Liner Type: Replaceable, pressed-in wet liners.
Bore Diameter: 150.0 mm (5.91 in).
Main Bearing Caps: Cast iron, four-bolt design on center caps.
The cylinder heads are constructed from cast iron,
matching the thermal expansion characteristics of the block to ensure reliable
head gasket sealing across the wide temperature range of heavy-duty diesel
operation. Separate castings are used for each cylinder bank, with each head
covering four cylinders. The OHV valvetrain operates sixteen valves - two per
cylinder - via pushrods and rocker arms, a configuration chosen for its
mechanical simplicity and ease of service.
The direct injection combustion system features a centrally located injector and
a bowl-in-piston combustion chamber design that promotes efficient air-fuel
mixing. Valve clearance is set manually using adjustment screws on the rocker
arms, a system requiring periodic maintenance. The valves are seated on hardened
steel inserts pressed into the cylinder head, with the valve seats ground to a
45-degree angle for optimal sealing.
Head Material: Cast iron.
Combustion Chamber Type: Direct injection with bowl-in-piston design.
Valve Configuration: OHV, 2 valves per cylinder with pushrods and rocker arms.
Valve Stem Diameter: 11.0 mm (0.43 in).
Valve Seat Angle: 45 degrees.
Valve Clearance (Cold, Intake): 0.40 mm (0.016 in).
Valve Clearance (Cold, Exhaust): 0.60 mm (0.024 in).
The 8M21 employs a mechanical direct injection fuel
system with an in-line injection pump - typically a Bosch P-series or
Mitsubishi-designed equivalent. The injection pump is mounted on the engine and
driven by the gear train, using individual plungers and barrels for each of the
eight cylinders. This design provides precise fuel metering and the high
injection pressures required for direct injection atomization.
High-pressure steel injection lines connect the pump to nozzle-type injectors
mounted centrally in each cylinder head. The injectors feature multi-hole
nozzles that deliver fuel directly into the piston bowl. A mechanical governor
regulates fuel delivery based on engine speed and load, while a manual primer
pump facilitates bleeding air after filter changes. A fuel filter with an
integrated water separator is mounted in the fuel supply line to remove
contaminants.
Injection Type: Mechanical direct injection.
Injection Pump: In-line type (Bosch P-series or equivalent).
Number of Plungers: 8.
Injection Timing: 12-14 degrees before top dead center.
Fuel Requirement: Diesel fuel with minimum cetane number of 45.
Fuel Filter: Replaceable cartridge type with integrated water separator.
The lubrication system is a fully pressurized, wet-sump
design engineered for the sustained high-load operation of heavy-duty diesel
engines. A gear-type oil pump, mounted to the front of the crankshaft, draws
lubricant from a large-capacity cast aluminum or stamped steel oil pan through a
mesh strainer. A full-flow, replaceable oil filter is mounted externally, with
an integral bypass valve that allows oil flow to continue if the filter becomes
restricted.
Pressurized oil is distributed through internal galleries to the crankshaft main
bearings, connecting rod bearings, camshaft bearings, and the timing gear train.
A water-cooled oil cooler is integrated into the system to stabilize oil
temperature under sustained high-load operation. The large oil capacity supports
extended service intervals in commercial applications.
Oil Type: Diesel-rated SAE 15W-40 or 10W-30 (API CD, CE, or higher).
Oil Capacity: 35-45 liters (37.0-47.5 US qt), depending on oil pan
configuration.
Oil Pressure (at rated speed): 294-441 kPa (42.6-64.0 psi).
Oil Pressure (at idle): 98 kPa (14.2 psi) minimum.
Oil Pump Type: Gear-type, crankshaft-driven.
The cooling system is a pressurized, liquid-cooled
design engineered to manage the substantial thermal load of a 21.2-liter V8
diesel engine in heavy commercial vehicle applications. A large-capacity
cross-flow radiator with a viscous-driven mechanical fan provides cooling
capacity for sustained high-load operation. The radiator core is sized to
maintain optimal engine temperatures under the maximum gross vehicle weight
conditions encountered in truck and bus service.
A centrifugal-type water pump, driven by the gear train, circulates coolant
through the cylinder block, cylinder heads, and the oil cooler. A wax-pellet
thermostat regulates coolant flow, with a bypass circuit that allows rapid
warm-up. The cooling system incorporates a coolant recovery tank to maintain
system pressure and prevent coolant loss.
Coolant Type: Ethylene-glycol based antifreeze with corrosion inhibitors.
Coolant Capacity: 14.0-16.0 liters (14.8-16.9 US qt) for the engine circuit.
Thermostat Opening Temperature: 76-82 °C (169-180 °F).
Thermostat Full-Open Temperature: 90 °C (194 °F).
Radiator Cap Pressure: 88-108 kPa (12.8-15.7 psi).
Water Pump Type: Centrifugal, gear train-driven.
| Intake and Exhaust Systems |
Intake System: Air is drawn through a heavy-duty
dry-type paper element air filter with a restriction indicator. The intake
manifold is a cast iron design with a large plenum chamber to ensure even air
distribution to all eight cylinders.
Exhaust System: Cast iron exhaust manifolds from each cylinder bank collect into
separate downpipes that merge into a single exhaust system. The exhaust system
is designed for the high exhaust gas temperatures encountered under sustained
full-load operation.
Timing Drive: The camshaft and injection pump are
driven directly by a train of precision-cut gears from the crankshaft. The gear
train is housed in a cast iron front cover and consists of a crankshaft gear,
idler gears, and camshaft gears for each cylinder bank. This gear-driven system
requires no periodic replacement and provides precise, permanent timing
synchronization.
Overall Length: 1,250 mm (49.2 in)
Overall Width: 950 mm (37.4 in)
Overall Height: 1,150 mm (45.3 in)
Dry Weight (complete engine): 1,150-1,250 kg (2,535-2,756 lb)
Oil Change Interval: 10,000 km (6,000 mi) under normal
commercial operating conditions; shorter intervals under severe conditions. Oil
Filter Change: At every oil change. Valve Adjustment Procedure: Clearance is set
by loosening the rocker arm lock nut and turning the adjustment screw.
Engine Coolant Replacement: 60,000 km (40,000 mi) or 2 years. Fuel Filter
Replacement: 20,000 km (12,000 mi) or as indicated by water separator
inspection. Fuel Water Separator Drain: Every 5,000 km (3,000 mi) to remove
accumulated water.
Glow Plug Inspection: 60,000 km (40,000 mi) for resistance and heating
performance on equipped variants. Valve Clearance Adjustment: Inspect and adjust
every 20,000 km (12,000 mi). Intake: 0.40 mm (0.016 in) cold. Exhaust: 0.60 mm
(0.024 in) cold.
Drive Belt (Accessory) Inspection: Every 10,000 km (6,000 mi) for tension,
cracking, and glazing. Air Filter Replacement: 30,000 km (18,000 mi) or as
indicated by restriction indicator. Compression Pressure: 2.75 MPa (399 psi) at
200 rpm.
Cylinder Head Bolts, 1st pass: 60 Nm (44 lb-ft).
Cylinder Head Bolts, 2nd pass: 120 Nm (89 lb-ft).
Cylinder Head Bolts, final pass: 175 Nm (129 lb-ft).
Main Bearing Cap Bolts, 1st pass: 60 Nm (44 lb-ft).
Main Bearing Cap Bolts, final pass: 115 Nm (85 lb-ft).
Connecting Rod Cap Bolts: 115 Nm (85 lb-ft).
Rocker Arm Shaft Bolts: 40-45 Nm (30-33 lb-ft).
Intake Manifold Bolts: 35-40 Nm (26-30 lb-ft).
Exhaust Manifold Nuts: 35-40 Nm (26-30 lb-ft).
Glow Plugs: 20-25 Nm (15-18 lb-ft).
Oil Pan Bolts: 20-25 Nm (15-18 lb-ft).
Water Pump Bolts: 30-35 Nm (22-26 lb-ft).
Injection Pump Mounting Nuts: 35-40 Nm (26-30 lb-ft).
Injection Line Nuts: 25-30 Nm (18-22 lb-ft).
Crankshaft Pulley Bolt: 350-400 Nm (258-295 lb-ft).
Flywheel Bolts: 315 Nm (232 lb-ft).
Front Gear Train Cover Bolts: 20-25 Nm (15-18 lb-ft).
The Mitsubishi 8M21 engine was utilized across
Mitsubishi Fuso’s heavy-duty commercial vehicle range and luxury coach
applications throughout the 1990s, primarily in the Japanese Domestic Market and
select export markets.
Mitsubishi Fuso Super Great (FP, FU, FV, FS series): Heavy-duty truck and
tractor applications from 1995 onward.
Mitsubishi Fuso The Great (late production): Heavy truck applications prior to
the Super Great introduction.
Mitsubishi Fuso Aero Queen: Luxury coach applications.
Mitsubishi Fuso Aero Ace: Luxury coach applications.
Mitsubishi Fuso Shogun: Export market heavy truck variants.
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